1. Field of the Invention
The present invention relates to a fuel cell stack formed by stacking a plurality of power generation cells in a direction of gravity. Each of the power generation cells includes an electrolyte electrode assembly and a separator stacked together. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. A reactant gas flow field is formed for allowing a reactant gas to flow along a surface of the separator. Reactant gas passages are formed for allowing the reactant gas to flow in the stacking direction.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (electrolyte electrode assembly) (MEA) which includes an anode, a cathode, and an electrolyte membrane (electrolyte) interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. The membrane electrode assembly is sandwiched between separators to form a power generation cell.
In automobile use, normally, in order to achieve a desired power generation output for automobile application, a predetermined number of (e.g., several tens to hundreds of) power generation cells are stacked together to form a fuel cell stack. In general, the fuel cell stack adopts so called internal manifold structure in which reactant gas flow fields for allowing reactant gases to flow along power generation surfaces in the separators, and reactant gas passages connected to the reactant gas flow fields and extending through the power generation cells in the stacking direction are provided.
In the fuel cell stack having the internal manifold structure, the water produced in the power generation reaction may be retained in the manifolds (reactant gas passages), in particular, on the outlet side. In this regard, for avoiding unstable operation due to flooding, for example, a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2004-146303 is known.
The fuel cell has a fuel cell stack formed by stacking unit cells, and each of the unit cells includes an ion-conductive electrolyte membrane, an anode and a cathode provided on both surfaces of the ion conductive electrolyte membrane, and a pair of electrically-conductive separator plates having a fuel gas flow field for supplying/discharging a fuel gas to/from the anode and an oxygen-containing gas flow field for supplying/discharging the oxygen-containing gas to/from the cathode.
Further, the cell stack includes a fuel gas inlet manifold for supplying the fuel gas to the fuel gas flow field, an oxygen-containing gas inlet manifold for supplying the oxygen-containing gas to the oxygen-containing gas flow field, a fuel gas outlet manifold for discharging the fuel gas from the fuel gas flow field, an oxygen-containing gas outlet manifold for discharging the oxygen-containing gas from the oxygen-containing gas flow field, a fuel gas inlet connected to the fuel gas inlet manifold, an oxygen-containing gas inlet connected to the oxygen-containing gas inlet manifold, a fuel gas outlet connected to the fuel gas outlet manifold, and an oxygen-containing gas outlet connected to the oxygen-containing gas outlet manifold.
Each of the manifolds extends through the electrically conductive separator plates in the stacking direction. At least one of the manifolds is downwardly-inclined in the gas movement direction. The inclination angle of the direction in which the manifold passes through the separator plate relative to the horizontal direction is not less than the angle at which droplets start to slide on the inner surface of the manifold, and not higher than 45°.
In Japanese Laid-Open Patent Publication No. 2004-146303, for example, when the oxygen-containing gas partially consumed in the power generation reaction and the water produced in the power generation reaction are discharged into the oxygen-containing gas outlet manifold of the electrically conductive separator plate, the water may flow around the end surface of the oxygen-containing gas outlet manifold, and the water may contact with another electrically-conductive separator plate. As a result, liquid junction (short circuit due to liquid) occurs between the unit cells (power generation cells). Then, corrosion occurs in the electrically conductive separator plates, and also the electrically conductive separator plates become thin.